An important commerical type of liquid level gauge designed to withstand the high pressures which exist in modern boilers such as are used for electric power generation, and which is adapted to be observed from a remote point through the agency of closed circuit television ("ITV") employs the so-called "bi-color" principle disclosed in Blackburn U.S. Pat. Nos. 2,024,815 and 2,115,899. The Blackburn type of gauge constituted a very important contribution to the art by reason of the fact that it virtually eliminated the danger that the observer might mistake a gauge full of steam for a gauge full of water--which error had on a number of occasions caused disastrous explosions.
Boiler codes require that the water level be directly observable by means which cannot give a false reading. In order to give a clear showing of the actual level, it is important that the entire gauge be illuminated. Where the gauge is of the multiple port type and the observation is by closed circuit television, it is common to place a display focusing device containing a series of refracting prisms between the gauge and the camera, to direct the light from the gauge ports more directly into the lens of the camera. However, the principle is the same in the sense that regardless of whether the gauge is observed directly or through such an interposed display device, illumination must be provided for the front of the display in addition to the light which is projected through the tube or ports of the gauge. Such frontal illumination is required in order to give a clear showing of the actual position of the water level.
Where observation is by ITV, as is usual, the frontal illumination requires a series of lamps which typically consumes between 600 and 700 watts. It is also the practice, where ITV observation is employed, to project white light through the gauge ports to the TV camera in the steam space, and in the water space to refract the white light laterally away from the line of view through the viewing ports, rather than using a polycolor gauge system, so that the steam space is shown brilliant white and the water space dark.
The conventional frontal illumination system inherently reduces the contrast between the illuminated steam-containing section and the surrounding gauge or display structure, but it will be recognized that if on the other hand adequate frontal illumination is not provided, particularly in the area of the dark, unilluminated water-containing section or ports, the relative length of the water-containing and steam-containing portions of the gauge is not quickly apparent.
The present invention aims to provide improved frontal illuminating means for such a gauge system, which creates increased contrast and so improves the readability of the gauge.
A related object is to provide such a system which concentrates the frontal illumination in the water-containing or dark port or tube region of the gauge system, so that in the steam area, above the water level, the front of the gauge or refraction screen is dark, but the ports are brilliantly illuminated, while in the water area, the ports are dark but the front of the gauge or display structure is illuminated. The total length of the system thus contains illuminated portions, but the contrast is increased.
Another object is to provide an improved liquid level gauge illumination system which is of increased reliability, which requires no separate source of light for frontal illumination, conserves energy, and is correspondingly less expensive to operate, but which nevertheless provides a further improvement in the contrast between the steam and water portions of the display.
Other objects and advantages of this invention will become apparent to those skilled in the art upon consideration of the present disclosure in its entirety.